.\" $Id: pvm_nrecv.3,v 1.1 1996/09/23 22:05:26 pvmsrc Exp $
.TH NRECV 3PVM "30 August, 1993" "" "PVM Version 3.4"
.SH NAME
pvm_nrecv \- Non-blocking receive.

.SH SYNOPSIS
.nf
.ft B
C	int bufid = pvm_nrecv( int tid, int msgtag )
.br

Fortran	call pvmfnrecv( tid, msgtag, bufid )
.fi

.SH PARAMETERS
.IP tid 0.8i
Integer task identifier of sending process supplied by the user.
.br
.IP msgtag
Integer message tag supplied by the user.
msgtag should be >= 0.
.br
.IP bufid
Integer returning the value of the new active receive buffer
identifier.  Values less than zero indicate an error.

.SH DESCRIPTION
The routine
.I pvm_nrecv
checks to see if a message with
label
.I msgtag
has arrived from
.I tid.
and also clears the current receive buffer if any,
If a matching message has arrived pvm_nrecv immediately
places the message in a new \fIactive\fR receive buffer,
and returns the buffer identifier in
.I bufid.
.PP
If the requested message has not arrived, then pvm_nrecv
immediately returns with a 0 in
.I bufid.
If some error occurs
.I bufid
will be < 0.
.PP
A -1 in
.I msgtag
or
.I tid
matches anything.
This allows the user the following options.
If tid = -1 and msgtag is defined by the user,
then pvm_nrecv will accept a message from any process
which has a matching msgtag.
If msgtag = -1 and tid is defined by the user,
then pvm_nrecv will accept any message that is sent from process tid.
If tid = -1 and msgtag = -1,
then pvm_nrecv will accept any message from any process.
.PP
The PVM model guarantees the following about message order.
If task 1 sends message A to task 2, then task 1 sends message B to task 2,
message A will arrive at task 2 before message B.
Moreover, if both messages arrive before task 2 does a receive,
then a wildcard receive will always return message A.
.PP
pvm_nrecv is non-blocking in the sense that the routine always returns
immediately either with the message or with the information
that the message has not arrived at the local pvmd yet.
pvm_nrecv can be called multiple times to check if a given
message has arrived yet.
In addition the blocking receive pvm_recv can be called for the same message
if the application runs out of work it could do before
the data arrives.
.PP
If pvm_nrecv returns with the message,
then the data in the message can be unpacked
into the user's memory using the unpack routines.

.SH EXAMPLES
.nf
C:
	tid = pvm_parent();
	msgtag = 4 ;
	arrived = pvm_nrecv( tid, msgtag );
	if ( arrived > 0)
		info = pvm_upkint( tid_array, 10, 1 );
	else
		/* go do other computing */
.sp
Fortran:
	CALL PVMFNRECV( -1, 4, ARRIVED )
	IF ( ARRIVED .gt. 0 ) THEN
	   CALL PVMFUNPACK( INTEGER4, TIDS, 25, 1, INFO )
	   CALL PVMFUNPACK( REAL8, MATRIX, 100, 100, INFO )
	ELSE
*	   GO DO USEFUL WORK
	ENDIF
.fi

.SH ERRORS
These error conditions can be returned by
.I pvm_nrecv.
.IP PvmBadParam
giving an invalid tid value or msgtag.
.IP PvmSysErr
pvmd not responding.
.PP
.SH SEE ALSO
pvm_bufinfo(3PVM),
pvm_getminfo(3PVM),
pvm_recv(3PVM),
pvm_unpack(3PVM),
pvm_send(3PVM),
pvm_mcast(3PVM)